1. Field of the Invention
The invention relates to apparatus for use in a long-haul echo control device, as well as accompanying methods for use therein, and particularly, though not exclusively, such a device which is intended for use with long-haul telecommunication facilities and which can automatically measure significant changes in length of a path and compensate for resulting variations in echo occurring over that path.
2. Description of the Prior Art
Echo is a common occurrence in a long haul telecommunications path. Echo typically arises owing to impedance mismatches at either end of that path, with its duration being a function of path length. In essence, as the path length increases, so does the duration of the echo.
Traditional echo control over an international telecommunications path that connects two international telecommunication administrations, and particularly a path that extends between two international switching centers (ISCs), has involved situating an echo control device (a so-called echo canceller) at each ISC. Each canceller controls the echo generated within its own corresponding domestic network. In those domestic networks that incorporate sophisticated echo cancellers, this approach functions quite well to control echo generated over connections occurring just within those networks.
However, for those telecommunications paths that extend from one ISC to another, users at one ISC, i.e., a “near-end ISC”, will depend on the performance of an echo canceller (a “far-end” echo canceller) situated at the far-end ISC. Consequently, that dependence has led to rather deficient performance.
First, in many countries, particularly in economically developing areas of the world, their telecommunications networks, particularly their ISCs, may not even contain echo cancellers. Second, what echo cancellers that do exist in those regions may function rather poorly or provide widely differing levels of performance. Consequently, long haul paths that rely on those ISCs are often plagued by echo.
To surmount these deficiencies, it is widely known in the art to position both near- and far-end echo cancellers, rather than at separate corresponding ISCs, at a single ISC, i.e., the near-end ISC. The near-end echo canceller controls locally generated echo; while, the far-end, or here long-haul, echo canceller controls echo generated at the far-end of the path—thus eliminating a need to rely on echo cancellers at the far-end ISCs. But even this approach carries its own limitations.
Specifically, when a long-haul path, such as a fiber cable, fails, that path could be restored over circuits that have markedly different path lengths and propagation delays than the failed cable; hence, yielding hard echo. For example, an undersea cable that suffered a break may be restored through a geostationary satellite link. While the cable may extend a few thousand miles between ISCs, a satellite link may extend on the order of 44,000 miles between ISCs (approximately 22,000 miles from one earth station to the satellite and a similar amount back down to a different earth station). This is the delay experienced by a listening party. The echo experienced by a speaking party is double this delay since the transmitted speech must once again traverse the same facility. In such instances, the widely different path lengths can result in substantial changes in echo delay. Unfortunately, international administrations rarely, if ever, provide any feedback information to each other regarding cable breaks or changes in path length. Therefore, an ISC situated at one-end of an international call rarely, if ever has knowledge, of changes in path length or delay as a result of call restoration events occurring at the other end.
Consider the following example. If the long haul facility consists of 11,200 miles (approximately 7000 km) of fiber cable between Administration A and B and Administration B's maximum internal practical facility mileage is on the order of 625 miles (approximately 1000 km), then round-trip delay for the far-end echo canceller is calculated as follows:Round Trip Long Haul Delay (LHD)=Total one-way distance (km)×(0.01 ms/1 km)where: 0.01 ms/km is a round trip delay factor per unit length of fiber facility.Here, the LHD=7000 km×(0.01 ms/1 km) or approximately 70 ms. As such, the far-end echo canceller must provide at least 70 ms of tail delay. In addition, within Administration B's network, the maximum Network Delay (ND) is calculated as follows:Round Trip maximum Network Delay (ND)=Total one-way distance (km)×(0.01 ms/1 km)Hence, ND=1000 km×(0.01 ms/1 km) or a maximum of 10 ms. Accordingly, the far-end echo canceller must provide between 70 to 90 ms of tail delay. This amount of delay can be readily provided by conventional echo cancellers. Now, let us assume that the long haul facility fails and the 7000 km fiber cable route is restored using a geostationary satellite. Since the satellite is approximately 22,000 miles in space (approximately 36,000 km) the facility mileage is now 72,000 km. Also since the satellite facility traverses air or free space instead of fiber cable, the round trip delay factor free space is 0.008 ms/km. Given this, the satellite LHD becomes 72,000 km×(0.008 ms/km) or 576 ms and a range of ND is 0 to 10 ms. The total round trip delay range is 576 to 586 ms. This amount of delay will produces rather hard echo for callers served by Administration A; thus significantly degrading call quality.
Consequently, in such installations, any significant resulting change in echo delay, such as from 80 ms to approximately 600 ms, must be handled by a technician at a near-end location who manually sets the long-haul echo canceller to accommodate that new delay value. But, once the failed path is restored, which could happen relatively quickly, the technician must then manually re-set the long-haul echo canceller back to its prior delay value. Not surprisingly, this approach, being highly labor-intensive, is rather impractical.
Therefore, a need exists in the art for an echo canceller, and particularly apparatus and accompanying methods for use therein, that can readily and automatically compensate for sufficient, e.g., significant, changes in long-haul path length and can be stationed along with a near-end echo canceller at a common end of the path. Advantageously, not only would such a resulting echo canceller provide proper response to changes in path length but also it would significantly reduce, if substantially eliminate, a need for manual intervention that would otherwise occur.